Markdown

Daring Fireball Projects: Markdown. From the author - "Markdown is a text-to-HTML conversion tool for web writers. Markdown allows you to write using an easy-to-read, easy-to-write plain text format, then convert it to structurally valid XHTML (or HTML)." Available as a Movable Type plugin or a standalone Perl script.

Also from Daring Fireball, Smarty Pants v 1.5 has been released.